The present invention relates to an accurate position measuring system utilizing a bidirectional data communication system which includes an artificial satellite. A conventional accurate position measuring system will now be described with reference to the accompanying drawings.
FIG. 9 schematically illustrates a conventional accurate position measuring system. The system includes a position measuring satellite 1, a receiving unit 2 and a position reference station 3. In this conventional system, the receiving unit 2 constitutes a position measuring device.
The position measuring satellite 1 transmits position measurement data necessary for position measurement calculation. The position reference station 3 previously stores its own exact position and calculates a positional error on the basis of the position of the station as calculated from the position measurement data transmitted from the satellite 1 and the previously stored exact position of the station. The position reference station 3 transmits the positional error as positional error information to the receiving unit 2. The receiving unit 2 receives the position measurement data from the satellite 1 and executes a position measurement calculation, with the calculation result being corrected in accordance with the positional error information from the position reference station 3.
Operation of the accurate position measuring system shown in FIG. 9 will now be described.
The position measurement data transmitted from the position measuring satellite 1 is received by the receiving unit 2 and the position reference station 3. The position reference station 3 calculates a positional error on the basis of the position of the station calculated by the received position measurement data and the exact position of the station stored previously, and transmits the positional error as the positional error information in an area where the position reference station 3 is located. In this case, the transmittable range of the positional error information from the position reference station 3 and the transmission frequency of the positional error information are determined, so that the receiving unit 2 positioned in the area can tune the predetermined frequency to receive the positional error information transmitted from the station 3 located in the area.
The receiving unit 2 uses the position measurement data received from the satellite 1 to perform the position measurement calculation so that the receiving unit calculates the position information relative to the current position thereof. Since the positional error of the current position is as relatively large as about 100 meters, the receiving unit 2 receives the position error information from the position reference station 3 and corrects the position information obtained by the position measurement calculation on the basis of the positional error information to thereby calculate the exact position information relative to the current position.
FIG. 10 is a functional block diagram of the receiving unit in the conventional accurate position measuring system. As shown in FIG. 10, the receiving unit 2 includes position measuring means 11, receiving means 12, frequency changing-over means 13, calculation means 14 and display means 15.
The position measuring means 11 receives the position measurement data from the position measuring satellite 1 and performs the position measurement calculation to calculate the position information relative to the current position. The receiving means 12 receives the positional error information from the position reference station 3. The frequency changing-over means 13 changes over the receiving frequency of the receiving means 12 in order to obtain the pertinent positional error information in accordance with the movement of the receiving unit. The calculation means 14 corrects the position information obtained by the position measuring means 11 on the basis of the positional error information received by the receiving means 12. The display means 15 displays the position information corrected by the calculation means 14.
Operation of the receiving unit 2 configured as discussed above will now be described.
The position measurement data transmitted from the position measuring satellite is received by the position measuring means 11 and the position information relative to the current position of the receiving unit 2 is calculated by the position measuring means 11. On the other hand, in order to obtain the positional error information corresponding to the area where the receiving unit 2 is positioned, a channel of the receiving means 12 is changed over to a frequency band assigned to the area by the frequency changing-over means 13, so that the receiving means 12 obtains the pertinent positional error information corresponding to the area where the receiving unit 2 is located. The positional error information received by the receiving means 12 is used to correct the position information calculated by the position measuring means 11, so that the position information has considerably high accuracy. The position information with high accuracy corrected by the calculation means 14 is displayed in the display means 15.
In the conventional accurate position measuring system described above, however, since the transmission frequency of the positional error information transmitted from the position reference station 3 is varied in accordance with the area where the position reference station 3 is located, it is necessary to change over the reception frequency of the receiving means 12 by the frequency changing-over means 13 each time the area is varied due to the movement of the receiving unit 2.
Further, since a broadcasting system which includes the position reference station 3 is used to transmit the positional error information, there are various problems in the broadcasting system due to the location and location intervals of the position reference stations 3, radio interference and the like.